OCI-5, the rat homologue of human glypican 3 (GPC3), is believed to be involved in morphogenesis and growth control during development. The finding that GPC3 is mutated in patients with the Simpson–Golabi–Behmel overgrowth syndrome is consistent with this idea. In this report, using RNAin situhybridization, expression of OCI-5 in the developing intestine is detected in both endoderm- and mesenchyme-derived cells in a phased manner related to age and proximal/distal position. To investigate the mechanism of its regulation during intestinal development, OCI-5 expression was studied in the primitive rat intestinal epithelial cell line IEC-18. The expression of the OCI-5 transcript is increased in IEC-18 cells at confluence, in low calcium media, and during spheroid culture, all conditions which result in the cells acquiring a more rounded cell shape. In contrast, cytoskeletal disruption with colchicine causes cells to flatten and spread and abolishes both the confluence- and the low calcium-dependent induction of OCI-5. Treatment with vanadate, a phosphatase inhibitor, causes cells to acquire a spindle-shaped morphology and prevents OCI-5 induction in all situations. Nuclear run-on analysis demonstrates that the rate of OCI-5 transcription is increased at confluence, in low calcium media, and during spheroid culture of IEC-18, and decreased by treatment of cells with colchicine. Together, these data suggest that OCI-5 expression is regulated in IEC-18 by cell shape. The pattern of expression of OCI-5 in the developing intestine is consistent with it playing a role in epithelial–mesenchymal interactions during intestinal morphogenesis, when cell shape changes are likely to occur.